Later today, DARPA will be testing the Falcon HTV-2 again, which mysteriously disappeared in its…
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After successfully separating from the missile, the Falcon reoriented itself for reentry using its Reaction Control System. During the reentry, it used RCS and its aero controls to fly into Earth's upper atmosphere, passing to the pull-up phase, which put it in the correct altitude for the glide phase. In theory, during this phase the Falcon was going to test its aerodynamics and integrity flying at Mach 20, experiencing temperatures of 3,500 degrees Fahrenheit—enough to melt steel.

DARPA controllers at Vandenberg acquired signal from reentry to some time into the glide phase. At that point, only 36 minutes after launch, they lost telemetry contact never to get it back again. According to DARPA, the Falcon has automated self-destruction controls in case something goes wrong, but they still don't know what has happened. Two hours ago at the time of this writing they haven't followed up on their last Tweet.

Back in April 2010, the first Falcon flew for nine minutes before DARPA experienced loss of signal. According to the Air Force, "the vehicle's onboard system detected a flight anomaly and engaged its onboard safety system-prompting the vehicle to execute a controlled descent into the ocean."

Why did the Falcon disappear again? To me, the answer is clear. In two words: